Unsubstituted diphenylamine can be used as an antioxidant, but it tends to yellow. A number of alkylated diphenylamines have shown good antioxidant activity in, for example, lubricating oils and polymeric molding compositions. These alkylated diphenylamines are typically low in yellow coloring and are resistant to further yellowing.
Antioxidant compositions have been formed by alkylating unsubstituted diphenylamine to produce a mixture of alkylated diphenylamines. Suitable alkylated diphenylamine compositions for use as antioxidants are liquids at room temperature (for example, 25.degree. C.) that contain no more than 3 wt. % unsubstituted diphenylamine, based on the total weight of substituted and unsubstituted diphenylamine in the alkylated diphenylamine composition.
Alkylation reactions of unsubstituted diphenylamine typically produce, among other reaction products, symmetrically disubstituted diphenylamines (e.g., dioctyldiphenylamine or dibutyldiphenylamine). Symmetrically disubstituted diphenylamines typically increase the melting point of the alkylated diphenylamine composition. In sufficient amounts (depending on the particular symmetrically disubstituted diphenylamine and the other components of the composition), the result is a solid alkylated diphenylamine composition rather than a liquid. For example, a composition with 25 wt. % or more dioctyldiphenylamine (based on the total weight of substituted and unsubstituted diphenylamine) is typically solid at room temperature. Thus, to obtain the desired liquid compositions with low amounts of unsubstituted diphenylamine, a balance must be found between forming symmetrically disubstituted diphenylamine and leaving unsubstituted diphenylamine in the composition.
The formation of alkylated diphenylamine compositions by reacting unsubstituted diphenylamine with diisobutylene is described in U.S. Pat. No. 5,750,787 and references cited therein. However, diisobutylene may not be sufficiently reactive to reduce the amount of unsubstituted diphenylamine in the alkylated diphenylamine composition to less than 3 wt. % in a suitable period of time and/or without increasing the amount of dioctyldiphenylamine in the alkylated diphenylamine composition above 25 wt. %, based on the total weight of substituted and unsubstituted diphenylamine in the alkylated diphenylamine composition.
A second olefin, which is more reactive with unsubstituted diphenylamine than diisobutylene under the reaction conditions, may be added to scavenge or reduce the amount of unsubstituted diphenylamine. Examples of such olefins include isobutylene, styrene, and .alpha.-methylstyrene. In conventional methods, the second olefin is added after the reaction of unsubstituted diphenylamine and diisobutylene is substantially complete or has proceeded to a particular point. According to the method described in U.S. Pat. No. 5,750,787, incorporated herein by reference, the desired alkylated diphenylamine composition is formed if the second olefin is added after the reaction between unsubstituted diphenylamine and diisobutylene has produced a reaction composition with less than 25 wt. % dioctyldiphenylamine, less than 25 wt. % unsubstituted diphenylamine, and greater than 50 wt. % monooctyldiphenylamine, based on the total weight of unsubstituted and substituted diphenylamine in the reaction composition.
The components of the alkylated diphenylamine compositions may affect other properties of the composition. For example, it is believed that both symmetric and asymmetrically disubstituted diphenylamines provide better color stability than monosubstituted diphenylamines when mixed with resins. Because symmetrically disubstituted diphenylamines increase the melting point of the composition and may result in the formation of solid, rather than liquid, compositions, the amount of symmetrically disubstituted diphenylamine(s) is typically limited. However, asymmetrically disubstituted diphenylamines, such as butyloctyldiphenylamine, have less effect on the melting point while typically providing the better color stability.